The overall objective of this project is to examine the factors that regulate DNA synthesis, cell division, and growth within the cell populations of embryonic and postnatal rat myocardia. The thrust of this project is to examine the regenerative potential of cardiac myocytes and attempt to define, in a more precise manner than has heretofore been the case, exactly what factors restrict cardiac cell proliferation in the adult, yet permit it in the embryo. The specific objectives of these studies are to elucidate and control those factors which inhibit cell replication and regeneration in aging mammalian heart; to reaggregate and reconstruct myocardia from dissociated heart cells at varying developmental ages in vitro, and to maintain these aggregates in the long-term culture to establish an in vitro model for studying cardiac regeneration; to establish a 'base-line' of competence for DNA synthesis and cell division of cardiac myocytes from hearts at increasing ages. An attempt will be made to establish whether DNA synthesis and cell division of embryonic and neonatal rat cardiac myocytes in vitro can be accelerated by the administration of insulin. The methods include preparation of cell suspensions by treatment of trypsin; aggregation-culture of cardiac myocytes, monolayer cell culture, light and electron microscopy; biochemistry, cytochemistry, and autoradiography.